This study seeks to test two etiologic hypotheses for chronic fatigue syndrome (CFS), a relatively common diagnosis of exclusion whose etiology is controversial. The primary hypothesis is that CFS occurs in some individuals whose dietary intake of sodium and potassium is inadequate. The secondary hypothesis is that autonomic dysfunction may interact with sodium depletion to produce CFS symptoms in some patients. The first stage of the study is a large dietary survey involving over 400 CFS patients and 200 age- and sex-matched controls who are interviewed three times by telephone. Dietary information is entered into the computers using the Minnesota Nutrition Data System. This study extensively utilizes the CDMAS facilities for all data input and telephone dietary surveys. Approximately 440 phone calls (each 30 minutes in length) have been completed.